Obesity is clearly one of the most visible public health problems in the US. Obesity now affects more than 30 % of the US adult population, increasing the risks for serious chronic diseases and reducing the life expectancy. This highlights urgent need to better understand the etiology of obesity and develop more effective therapies against obesity. Leptin is a key adipocyte-derived hormone that potently suppresses food intake, reduces body weight, and increases energy expenditure. Thus, leptin was once thought to be a magic bullet for the treatment of obesity. However, leptin does not work in obese people because of the development of leptin resistance. This observation creates one of the fundamental questions to be addressed in the field: what are the mechanisms of neuronal leptin resistance, a hallmark of human obesity. To address this question, we have been using an organotypic brain slice model as an in vitro tool to investigate the molecular mechanisms underlying cellular leptin resistance in hypothalamic neurons, a primary site of leptin action. Using this tool, we initially found that the cAMP-related pathway potently induces leptin resistance through Epac-Rap1 signaling. Epac is an exchange factor for GTP/GDP for the small G protein Rap1. Furthermore, we have searched for an extracellular upstream factor(s) that induces both the activation of Epac-Rap1 signaling and leptin resistance. To this end, we have been conducting a candidate-ligand approach based on the fact that Epac-Rap1 signaling can be activated by a variety of G protein-coupled receptors (GPCRs) that produce cAMP. We have been systematically screening the ligands of known GPCRs that couple to cAMP signaling. During our initial screening, we have identified the gut hormone glucose-dependent insulinotropic polypeptide (GIP) as a promising candidate. Based on these previous observations and our preliminary data, we hypothesize that the gut-derived GIP acts as a previously unrecognized circulating signal that drives neuronal leptin resistance via directly activating Epac-Rap1 signaling during obesity. In our specific aims, in Aim1, we will use a Cre- dependent conditional Rap1 knockout mouse to determine the physiological relevance of Rap1 expressed by leptin responsive neurons in diet-induced leptin resistance and obesity. In Aim2, we will determine if GIP receptor in leptin responsive neurons is required for diet-induced leptin resistance and obesity by using GIP receptor deficient mice. In Aim3, through a combination of pharmacological and genetic studies in rodents, we will also explore potential intervention strategies targeting GIP receptor and Epac, and using the same approach, we will determine the molecular mechanisms mediating the effect of brain GIP-Epac-Rap1 signaling on modulating leptin sensitivity. The results of these studies are potentially paradigm-shifting, and should provide a framework for a better understanding of central leptin resistance.